The present invention relates to method and a system for controlling an interstand tension in a continuous rolling mill.
Most important factors for evaluating the quality of products rolled in the continuous rolling mills are thickness and width of sheet; the amount of crowning and flatness. These factors are greatly affected by an interstand tension i.e., a tension exerted on the workpiece passing between two successive rolling stands. The variation in the interstand tension must therefore be kept as small as possible. For this reason, hot continuous rolling mills are provided with a looper between successive rolling stands to minimize variation in the tension. In addition, the speed of the rolling stand adjacent the looper is controlled to minimize the range of angle over which the looper swings.
FIG. 1 shows an example of a conventional tension control system. A workpiece 1 to be rolled passes through a rolling stand 2 having a pair of working rolls 2a and 2b, engages with a looper 4 and then passes through another rolling stand 3 having a pair of working rolls 3a and 3b. The looper 4 is of a type with which the looper torque given by a looper drive motor 8 is in balance with the tension on the workpiece 1. The operating angle .theta., i.e., the angle between the axis of the arm 4a of the looper 4 and an imaginary horizontal line 4b is detected by a looper angle sensor 5. The detected angle .theta. is applied to an operation unit 6 which computes, in accordance with the detected angle .theta., the torque to maintain the tension at a desired value. More particularly, the detected angle .theta. is used for the calculation by the looper operation unit 6 to determine the target value of the electric current of the looper drive motor 8, and the target value is supplied to a current control device 7 which drives the looper drive motor 8.
The output from the looper angle sensor 5 is also fed to an operation unit 9 which computes a target value of the speed of a roll drive motor 11 to return the angle .theta. to a desired value. The speed target value is applied to a speed control device 10 which controls the speed of the roll drive motor 11.
The control system described above, however, has the following drawbacks. First, since a current control device 7 is used to control the looper drive motor 8, a circuit for compensation for stabilization is required, and it is necessary to compute the initial current target value.
Secondly, the change of the speed of the roll drive motor 11 results in variation of the length of the workpiece 1 between the successive rolling stands 1 and 2, following which the looper angle control is made. As a result, considerable variations occur in the tension on the workpiece.
Thirdly, making the looper angle control to reduce the variation in tension necessitates lowering the response of control. This degrades the capability of the system to follow rapidly changing disturbance.